A Local Habitation (October Daye series 2, by Seanan McGuire)

This is going to be brief (for once). The series is still interesting. Toby is still creating "Oh, Toby!" moments, but this time, she's joined in her faux pas by most of the cast. Toby's liege sends her after his missing (adult) niece, who is in charge of a small independent Faerie political unit named Tamed Lightning. Most of Tamed Lightning is a computer company, and it turns out that the place is hosting a serial killer: especially problematic for Fae, who otherwise can expect to live forever. What follows is a semi-locked room mystery, because communications in and out of Tamed Lightning are ... problematic.

( Cut for lots of spoilers )